The transmissivity of light through an optical fiber is highly dependent on the properties of the coatings applied to the fiber. The coatings typically include a primary coating and a secondary coating, where the secondary coating surrounds the primary coating and the primary coating contacts the glass fiber (which includes a central glass core surrounded by a glass cladding). The secondary coating is a harder material (higher Young's modulus) than the primary coating and is designed to protect the glass waveguide from damage caused by abrasion or external forces that arise during processing, handling, and installation of the fiber. The primary coating is a softer material (low Young's modulus) and is designed to buffer or dissipates stresses that result from forces applied to the outer surface of the secondary coating. Dissipation of stresses within the primary layer attenuates the stress and minimizes the stress that reaches the glass waveguide. The primary coating is especially important in dissipating stresses that arise when the fiber is bent. The bending stresses transmitted to the glass waveguide on the fiber needs to be minimized because bending stresses create local perturbations in the refractive index profile of the glass waveguide. The local refractive index perturbations lead to intensity losses for the light transmitted through the waveguide. By dissipating stresses, the primary coating minimizes bend-induced intensity losses.
To minimize bending losses, it is desirable to develop primary coating materials with increasingly lower Young's moduli. Coating materials with a Young's modulus below 1 MPa are preferred. As the Young's modulus of the primary coating is reduced, however, cohesion of the primary coating deteriorates and the primary coating is more susceptible to damage in the fiber manufacturing process or during deployment in the field. Operations such as stripping, cabling, and connecting introduce thermal and mechanical stresses to the primary coating that can that lead to the formation of defects in the primary coating. The formation of defects in the primary coating becomes more problematic as the cohesion of the primary coating decreases.
In addition to good cohesion, fiber stripping and splice operations require primary coatings with proper adhesion to the glass fiber. If the adhesion of the primary coating to the glass fiber is too strong, residue from the primary coating remains on the glass fiber and it is difficult to achieve a clean strip. A clean strip is needed to insert the glass fiber into a connector. The opening in fiber connectors is closely matched to the diameter of the glass fiber and the presence of coating residue on the glass fiber prevents insertion of the fiber into a connector. The adhesion requirements are particularly stringent for ribbons, which are linear fiber assemblies that include multiple fibers in a common matrix. When connecting ribbons, all fibers are stripped simultaneously and each fiber must be stripped cleanly without introducing defects in the coating remaining on the unstrapped portion of the glass fiber.
There is a need for a primary coating material that can be cleanly stripped from glass fibers while also having sufficient cohesion to resist formation of defects when the stripping force is applied.